The present invention pertains generally to electric plasma thrusters and more particularly to Hall field thrusters, which are sometimes called Hall accelerators.
The Hall plasma accelerator is an electrical discharge device in which a plasma jet is accelerated by a combined operation of axial electric and magnetic fields applied in a coaxial channel. The conventional Hall thruster overcomes the current limitation inherent in ion diodes by using neutralized plasma, while at the same time employing radial magnetic fields strong enough to inhibit the electron flow, but not the ion flow. Thus, the space charge limitation is overcome, but the electron current does draw power. Hall thrusters are about 50% efficient. Hall accelerators do provide high jet velocities, in the range of 10 km/s to 20 km/s, with larger current densities, about 0.1 A/cm2, than can conventional ion sources.
Hall plasma thrusters for satellite station keeping were developed, studied and evaluated extensively for xenon gas propellant and jet velocities in the range of about 15 km/s, which requires a discharge voltage of about 300 V. Hall thrusters have been developed for input power levels in the general range of 0.5 kW to 10 kW. While all Hall thrusters retain the same basic design, the specific details of an optimized design of Hall accelerators vary with the nominal operating parameters, such as the working gas, the gas flow rate and the discharge voltage. The design parameters subject to variation include the channel geometry, the material, and the magnetic field distribution.
A. V. Zharinov and Yu. S. Popov, xe2x80x9cAcceleration of plasma by a closed Hall currentxe2x80x9d, Sov. Phys. Tech. Phys. 12, 1967, pp. 208-211 describe ideas on ion acceleration in crossed electric and magnetic field, which date back to the 1950""s. The first publications on Hall thrusters appeared in the United States in the 1960""s, such as: G. R. Seikel and F. Reshotko, xe2x80x9cHall Current Ion Acceleratorxe2x80x9d, Bulletin of the American Physical Society, II (7) (1962) and C. O. Brown and E. A. Pinsley, xe2x80x9cFurther Experimental Investigations of Cesium Hall-Current Acceleratorxe2x80x9d, AIAA Journal, V.3, No 5, pp. 853-859, 1965.
Over the last thirty years, A. I. Morozov designed a series of high-efficiency Hall thrusters. See, for example, A. I. Morozov et al., xe2x80x9cEffect of the Magnetic field on a Closed-Electron-Drift Acceleratorxe2x80x9d, Sov. Phys. Tech. Phys. 17(3), pp. 482-487 (1972), A. I. Morosov, xe2x80x9cPhysical Principles of Cosmic Jet Propulsionxe2x80x9d, Atomizdat, Vol. 1, Moscow 1978, pp. 13-15, and A. I. Morozov and S. V. Lebedev, xe2x80x9cPlasma Opticsxe2x80x9d, in Reviews of Plasma Physics, Ed. by M. A. Leontovich, V.8, New York-London (1980).
H. R. Kaufman, xe2x80x9cTechnology of Closed Drift Thrustersxe2x80x9d, AIAA Journal Vol. 23 p. 71 (1983), reviews of the technology of Hall field thrusters, both in the context of other closed electron drift thrusters and in the context of other means of thrusting plasma. V. V. Zhurin et al., xe2x80x9cPhysics of Closed Drift Thrustersxe2x80x9d, Plasma Sources Science Technology Vol. 8, p. R1 (1999), further reviews the physics and more recent developments in the technology of Hall thrusters.
What remains a challenge is to develop a Hall thruster able to operate efficiently at low power. To reduce the cost of various space missions, there is a strong trend towards miniaturization of satellites and their components. For some of these missions, which use on board propulsion for spacecraft orbit control, this miniaturization requires development of micro electric thrusters, having a large specific impulse (1000-2000 sec), which can operate efficiently at low input power levels, that is, less than 200 watts. However, existing small Hall thrusters, which are simply scaled down by means of a linear scaling to operate at low input power, are very significantly less efficient than Hall thrusters operating at input power larger than 0.5 kW.
The conventional annular design is not well suited to scaling to small size, because the small size for an annular design has a great deal of surface area relative to the volume. A more sensible design at small size would be a cylindrical geometry design. A cylindrical design may also be useful at high power, but it may be technologically indispensable for Hall field acceleration at low power.
The present invention comprises an improvement over the prior art cited above by providing for efficient operation of a cylindrical geometry Hall thruster, in which the center pole piece of the conventional annular design thruster is eliminated or greatly reduced. The present invention discloses means of accomplishing efficient operation of such a thruster by designing magnetic fields with a substantial radial component, such that ions are accelerated in substantially the axial direction.
The present invention comprises an improvement as well as over the following prior art:
U.S. Pat. No. 4,862,032 (xe2x80x9cEnd-Hall ion sourcexe2x80x9d, Kaufman et al., Aug. 29, 1989) discloses specifically that the magnetic field strength decreases in the direction from the anode to the cathode. The disclosure of the above referenced patent is hereby incorporated by reference.
Other design suggestions are disclosed in U.S. Pat. No. 5,218,271 (xe2x80x9cPlasma accelerator with closed electron driftxe2x80x9d, V. V. Egorov et al., Jun. 8, 1993) which contemplates a curved outlet passage. The disclosure of the above referenced patent is hereby incorporated by reference. U.S. Pat. No. 5,359,258 (xe2x80x9cPlasma accelerator with closed electron driftxe2x80x9d, Arkhipov et al., Oct. 25, 1994) contemplates improvements in magnetic source design by adding internal and external magnetic screens made of magnetic permeable material between the discharge chamber and the internal and external sources of magnetic field. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,475,354 (xe2x80x9cPlasma accelerator of short length with closed electron driftxe2x80x9d, Valentian et al., Dec. 12, 1995) contemplates a multiplicity of magnetic sources producing a region of concave magnetic field near the acceleration zone in order better to focus the ions. The disclosure of the above referenced patent is hereby incorporated by reference. U.S. Pat. No. 5,581,155 (xe2x80x9cPlasma accelerator with closed electron driftxe2x80x9d, Morozov, et al., Dec. 3, 1996) similarly contemplates specific design optimizations of the conventional Hall thruster design, through specific design of the magnetic field and through the introduction of a buffer chamber. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,763,989 (xe2x80x9cClosed drift ion source with improved magnetic fieldxe2x80x9d, H. R. Kaufman Jun. 9, 1998) contemplates the use of a magnetically permeable insert in the closed drift region together with an effectively single source of magnetic field to facilitate the generation of a well-defined and localized magnetic field, while, at the same time, permitting the placement of that magnetic field source at a location well removed from the hot discharge region. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,847,493 (xe2x80x9cHall effect plasma acceleratorxe2x80x9d, Yashnov et al., Dec. 8, 1998) proposes that the magnetic poles in an otherwise conventional Hall thruster be defined on bodies of material which are magnetically separate. The disclosure of the above referenced patent is hereby incorporated by reference.
U.S. Pat. No. 5,845,880 (xe2x80x9cHall effect plasma thrusterxe2x80x9d, Petrosov et al., Dec. 8, 1998) proposes a channel preferably flared outwardly at its open end so as to avoid erosion. The disclosure of the above referenced patent is hereby incorporated by reference.
It is an object of this invention to provide a cylindrical Hall plasma thruster, made efficient by means of detailed control of the magnetic and electric fields.
The cylindrical configuration consists of a cylindrical ceramic channel, in which there is imposed a magnetic field strong enough to impede the motion of the electrons but not the motion of the ions. The imposed magnetic field is substantially axial near the gas entrance and substantially radial near the gas exit. The invention exploits the fact that, as in a conventional Hall thruster, the lines of magnetic force substantially form equipotential surfaces, so that where the magnetic field is radial, the electric field is axial and where the magnetic field is axial the electric field is radial. Electrons are therefore impeded axially, and tend to drift in the azimuthal direction about the cylinder axis, whereas ions are accelerated radially where the magnetic field is substantially axial and axially where the magnetic field is substantially radial.
The invention utilizes appropriate magnetic circuits to enlarge the region in which the magnetic field is largely radial, and therefore the acceleration of the ions is largely axial. The invention discloses means for the gas to be preferentially ionized near those regions, so that substantial axial acceleration of ions results.
The present invention is a new kind of Hall thruster, which has a cylindrical ceramic channel and at least two magnetic poles. One magnetic pole is located on the thruster axis on the back wall of the channel, or slightly in front of the back wall of the channel. The other pole is located at the open exit of the channel. This design may be understood with reference to FIG. 1. As shown in FIG. 1, such a magnetic circuit produces a magnetic field that is substantially axial near the gas inlet, and is substantially radial after a region of ionization. In the vicinity of the radial magnetic field, ions undergo substantially axial acceleration as they do in a conventional Hall thruster geometry, with an annular channel design.
We disclose herein methods of producing an electric potential profile, an ionization profile, and a magnetic field profile such that ions tend to be accelerated axially.
In the simplest embodiment of the present invention, the electric potential profile is established without detailed control between the anode 7 and hollow cathode 8. Gas is input in the vicinity of the anode and then ionized through impact with energetic electrons. There will be substantial acceleration of the ions in the axial direction because the magnetic field lines support a potential drop in the axial direction. The axial acceleration occurs where the magnetic field is radial. Therefore the magnetic field is arranged to have a substantial radial component.
In order to enhance the acceleration of the ions in the axial direction, we disclose that the ionization region can be arranged to occur substantially near where the magnetic field lines are radial, rather than where the magnetic field lines are axial. To do so, the gas may be introduced into the channel away from the channel mid-plane. Thus, the anode 7, which can also be a gas distributor, can have an annular geometry.
In order to further control the axial acceleration of the ions, segmented electrodes 26 can be introduced along the channel walls (see FIG. 2). Since each magnetic field line is substantially at the same electric potential, because electrons can freely more along the field line, the introduction of segmented electrodes in the ceramic channel wall can define a potential drop between any two magnetic field lines. We disclose an efficient means of axially accelerating said ions occurs by thus arranging the main potential drop in the region where the radial magnetic field is closest to the ionization region.
As a further enhancement of the axial acceleration of the ions, we disclose that the magnetic field can be made more nearly radial through the introduction of a second magnetic coil 6 (see FIG. 3). The second coil also provides for focusing of the ion stream.
In a preferred embodiment of the invention, the magnetic pole 10 can protrude somewhat in front of the anode (see FIG. 4). This produces a very small annular region in front of the gas distributor. In this annular region the magnetic field is almost purely radial. This annular region then serves as an enhanced ionization region. To further enhance the axial acceleration, segmented electrodes can be used as well in this configuration (see FIG. 5).
In a preferred embodiment, the front of the magnetic pole 10 can serve as an excellent location for a cathode-neutralizer. In FIG. 6, we disclose the use of a segmented emissive electrode 18 placed in the ceramic channel 12 in front of the magnetic pole piece 10. This emissive electrode 18 can provide electrons in a region where they will not be impeded by the magnetic field as they escape from the thruster. Therefore, this emissive electrode can provide electrons to neutralize the accelerated ions. In yet a further preferred embodiment, segmented emissive electrode 18 can replace entirely the cathode neutralizer 8.